1. Field of the Invention
The present invention relates to a photomask, and more specifically, to a photomask used when a pattern is transferred to a semiconductor substrate.
2. Description of the Background Art
As semiconductor integrated circuits are miniaturized, a phase shift technology as one of methods of improving resolution of a fine pattern has been developed, which is a method of inverting by 180.degree. a phase of light transmitted through light transmitting regions formed adjacent to each other on a photomask for weakening an interference with light to each other to enhance the resolution.
FIG. 1O is a sectional view of a photomask which is used in transferring a pattern to a semiconductor substrate, utilizing the phase shift technology. In FIG. 10, the photomask includes a transparent glass substrate 1 of composite quartz of 0.09 inch thickness and of 90% light transmittance, an antireflection film 2 formed of molybdenum oxide silicide of 400 .ANG. thickness on a first major surface of the glass substrate 1, a non-transparent film 3 formed of molybdenum silicide of 1000 .ANG. thickness on the antireflection film 2, an antireflection film 4 formed of molybdenum oxide silicide of 400 .ANG. thickness on the non-transparent film 3, a phase-shifting film 5 formed of silicon oxide of 4000 .ANG. thickness and of 90% light transmittance on the first major surface of the glass substrate 1.
The non-transparent film 3 shuts out light, and the antireflection films 2, 4 prevent light from being reflected to top and bottom surfaces of the non-transparent film 3. The non-transparent film 3 and the antireflection films 2, 4 are formed on the same pattern, and based upon the pattern, resist on the semiconductor substrate is irradiated with light. The shifter film 5 delays the light so that it arrives 180.degree. out of phase with the incident light.
FIG. 11 is a schematic diagram showing a phase shift technology in which the photomask of FIG. 10 is used. First, the glass substrate has its second major surface irradiated with g-lines of 436 nm wavelength. The g-lines are transmitted through all the regions except for the non-transparent film 3, that is, through the glass substrate 1 alone, or both the glass substrate 1 and the shifter film 5. The g-lines transmitted through the glass substrate 1 and the shifter film 5 is 180.degree. out of phase with the g-lines transmitted through the glass substrate 1 alone. Thus, when a positive-type photoresist coating the semiconductor substrate is irradiated with the g-lines transmitted through the photomask, the amplitude of the g-lines on the wafer periodically varies in positive and negative directions. Then, when the surface of the positive-type photoresist which has been irradiated with the g-lines is removed by development, the intended pattern can be obtained.
In the schematic diagram of FIG. 11 showing the phase shift technology, the g-lines transmitted through the glass substrate 1 alone are decayed in intensity to 90% of the intensity of the g-lines directed to the first major surface of the glass substrate 1. Also, the g-lines transmitted through the glass substrate 1 and the shifter film 5 are decayed to 80% of the intensity of the g-lines directed to the first major surface of the glass substrate 1 (First, the glass substrate 1 attenuates the g-lines to 90% of its initial, and the shifter film 5 further attenuates to 90%. Thus, the sum of the attenuation is 90%.times.90%.apprxeq.80%). Hence, when the positive type photoresist coating the semiconductor substrate is irradiated with the g-lines transmitted through the photomask, the peak intensity on the photoresist is alternately to be 80% or 90% of the incident g-lines on the predetermined cycle. Thus, in the region irradiated with the g-lines having the 90% peak intensity, a developing rate is higher than that in the region irradiated with the g-lines having the 80% peak intensity, and consequently, adjacent grooves A and B defined by the pattern become different in width. Thus, there arises the problem that an accurate pattern cannot be obtained.